【Introduction】Microphones are already standard devices built into many Electronic products, from wearables to home assistants, more and more devices are required to “hear” their environment and respond accordingly. This article will provide a comprehensive overview of microphone types and fundamentals, as well as product features of CUI Devices Micro Electro Mechanical Systems (MEMS) microphones.
Technical differences between ECM and MEMS microphones
With the increase of microphone applications, the requirements for the sensitivity and volume of the microphone are also getting higher and higher. The two most common technologies used to build microphones today are MEMS and electret capacitors. The following will first introduce the basics of MEMS and electret condenser microphones (ECM), compare the differences between the technologies, and outline the solutions for each program advantages.
MEMS microphones consist of MEMS components placed on a printed circuit board (PCB) and protected by a mechanical cover. It makes a small hole in the housing to allow sound into the microphone, designated as the top port if the opening is in the top cover, and the bottom port if the opening is in the PCB. MEMS components are typically designed with mechanical diaphragms and mounting structures created on semiconductor chips.
The MEMS diaphragm forms a capacitor, and sound pressure waves cause the diaphragm to move. MEMS microphones typically contain a second semiconductor chip that acts as an audio preamplifier, converting the MEMS’ changing capacitance into an electrical signal. If an analog output signal is required, the output of the audio preamp is provided to the user. If a digital output signal is required, an analog-to-digital converter (ADC) will be included on the same chip as the audio preamp. A common format for digital encoding used in MEMS microphones is pulse density modulation (PDM), which allows communication with only a clock and a single data line. Digital signal decoding at the receiver is simplified due to the single-bit encoding of the data. The digital I²S output is a third option and includes an internal decimation filter, which allows processing to be done in the microphone itself. This means that the microphone can be connected directly to a digital signal processor (DSP) or microcontroller, eliminating the need for ADCs or codecs in many applications.
The ECM consists of an electret diaphragm (a material with a fixed surface charge) separated and adjacent to a conductive plate, similar to a MEMS microphone, forming a capacitor with an air gap serving as the dielectric. As the sound pressure wave moves the electret diaphragm, the Voltage across the capacitor varies with the capacitance value, which is amplified and buffered by the JFET inside the microphone housing. JFETs are usually configured in a common source configuration, while external load resistors and DC blocking capacitors are used in the external application circuit.
ECM and MEMS microphones have their own advantages
There are many factors to consider when choosing between ECM and MEMS microphones, and the market share of MEMS microphones continues to grow rapidly due to the many advantages offered by this new technology. For example, space-constrained applications are attractive to the small package size of MEMS microphones, which can reduce PCB area and component cost due to the inclusion of analog and digital circuitry in the MEMS microphone structure. The output impedance of analog MEMS microphones is relatively low, while the output of digital MEMS microphones is well suited for applications in electrically noisy environments. In high vibration environments, the use of MEMS microphone technology can reduce unwanted noise levels introduced by mechanical vibrations. In addition, the addition of semiconductor fabrication techniques and audio preamplifiers has made it possible to manufacture MEMS microphones with closely matched and temperature-stable performance characteristics. These stringent performance characteristics are especially beneficial when MEMS microphones are used in array applications. MEMS microphones are also easily handled by pick and place machines during product manufacturing and can withstand the reflow soldering temperature profile.
Although MEMS microphones are rapidly gaining popularity, there are still some applications where ECM may be preferred. Many legacy designs use ECM, so if the project is a simple upgrade to an existing design, it’s best to keep using ECM. Options for connecting the ECM to the application circuit include pins, wires, SMT, pads, and spring contacts, giving engineers additional design flexibility. If dust and moisture resistance is an issue, it is easy to find ECM products with high protection (IP) ratings due to their larger physical size. For projects requiring non-uniform spatial sensitivity, ECM products can provide unidirectional or noise cancellation with inherent directionality, and the wide operating voltage range of ECMs may be the preferred solution for products with loosely regulated voltage rails.
PDM and I²S protocols have different characteristics
In addition to a significantly reduced footprint, lower power requirements, and greater electrical noise rejection, one of the key benefits of MEMS microphones is the increased output options that provide designers and engineers with greater flexibility . While analog options are still available, two popular output options are the digital protocols of PDM and I²S. Each of these interfaces has its own unique characteristics, and key factors to consider include audio quality levels, power consumption levels, bill of materials cost, space constraints the design must adhere to, and the operating environment in which the hardware will be deployed.
PDM is used to convert an analog signal voltage into a unit pulse density modulated digital stream. PDM signals look more like longitudinal waves than the typical shear waves associated with audio, but they are a digital representation of an analog signal. This produces a signal that has a digital signal. many advantages, while still being directly related to analog signals. Creating this PDM signal requires a higher sampling rate than usual (a rate higher than 3 MHz) because the digital pulses must occur at a frequency many times higher than the oscillation frequency of the analog signal they represent.
Due to the digital nature of the signal, PDM is more resilient to electrically noisy environments than analog signals and has a higher error tolerance when the signal is degraded. High-frequency signals do create distance limitations because of the increased capacitance on longer transmission lines, which can cause unwanted attenuation and accompanying degradation in audio quality. These signals also need to be further processed by an external DSP or microcontroller with an appropriate codec to run the PDM signal through a low pass filter to decimate or downsample it to a lower sample rate, making it usable for other device.
Unlike PDM, I²S is a fully digital signal that does not require encoding or decoding, and there is no universally required data transfer speed, however, the minimum speed depends on the data being transferred and its precision. If the audio sample rate is the industry standard of 44.1 kHz and the precision is 8 bits, then a mono channel will require a clock speed of at least 352.8 kHz. Stereo applications will be double that of 705.6 kHz, and any change in precision will also change the minimum transmission bandwidth.
PDM provides better noise immunity and bit error tolerance, which can make it attractive for many applications where audio quality is a priority. In contrast, I²S is easier to install, reduces the overall footprint and reduces the number of components, which will have advantages where product size or its price tag proves to be a major concern. It should also be noted that the I²S interface will provide better signal integrity over longer distances, so it is also suitable for situations where the microphone and processing circuitry cannot be placed close to each other on the board.
Provide a variety of MEMS microphone selectivity
MEMS microphones have become commonplace in modern electronic design, and having the best possible interface is critical. There are many factors to consider when deciding which interface will optimize your specific use case. PDM can be ideal in challenging application environments thanks to its inherent noise immunity. Conversely, using I²S allows the input to be connected directly to its accompanying DSP or other processor/controller device without any additional complexity.
CUI Devices has a broad portfolio of MEMS microphones to meet various audio system requirements. In addition to analog interface units, a variety of PDM and I²S digital interface microphones are also available. CUI Devices’ MEMS microphones offer users better audio quality and performance in compact, low-profile packages as small as 2.75 x 1.85 x 0.90 mm. With a sensitivity rating of -42 dB to -26 dB, a signal-to-noise ratio of 57 dBA to 65 dBA, and a sensitivity tolerance as low as ±1 dB, these MEMS microphones are ideal for a range of portable consumer electronics applications. For easier prototyping and design testing, CUI Devices also offers a MEMS microphone development kit that includes four separate microphone evaluation circuits.
CUI Devices’ MEMS microphones are available in top and bottom port versions, analog and digital options, and sensitivity levels from -42 dB to -26 dB, with current consumption as low as 80 µA, you can find the best MEMS microphone for your needs at:
MEMS microphones have the advantages of small size and strong anti-noise ability, and are the first choice for many consumer application products. CUI Devices provides product types such as different sensitivities and interfaces, and provides complete design resources, which can help customers develop corresponding products quickly. It is worthwhile You learn more and adopt.